A contactless communication system called RFID (Radio Frequency Identification) is known as a communication system in which a communication terminal not having a generating source of radio waves for itself transmits data to a device to be the other party of communication by wireless. The RFID is also called as an ID system, a data carrier system and the like, however, the RFID system which is abbreviated as RFID is common worldwide. The RFID means an “identification system using a high frequency (radio wave)”.
The RFID system is applied to various contactless IC cards. An IC card system includes an IC (Integrated Circuit) card as a transponder and a device performing reading information from the IC card or writing of information to the IC card (referred to as a “reader/writer” in the following description). Such IC card system is convenient because reading/writing of information is performed in a contactless manner between the IC card and the reader/writer. The reader/writer is a device which starts interactive communication by outputting an electromagnetic wave first (that is, the device which takes initiative of communication), which is also referred to as an “initiator”. The transponder such as the IC card is a “target” which sends a response (interactive communication start response) with respect to a command (interactive communication start request) from the initiator. In a passive mode, a carrier signal is constantly directed from the initiator to the target, whereas in the active mode, the carrier signal is switched alternately. In the following description, communication from the reader/writer to the transponder will be called a “down link” and communication from the transponder to the reader/writer is called “uplink”.
As contactless communication methods which can be applied to the RFID, an electrostatic method, an electromagnetic induction method, a radio wave communication method and the like can be cited. Among them, the electromagnetic induction method includes a primary coil on the reader/writer side and a secondary coil on the card (or the transponder) side, in which data communication is performed via the coils by magnetic coupling of these two coils. Specifically, the reader/writer transmits data by performing amplitude modulation on a magnetic field generated by the primary coil, and the transponder side detects the data. The transponder also performs modulation processing such as amplitude modulation by load switching (LS) of the secondary coil to thereby transmit data to the reader/writer. Each coil of the transponder and the reader/writer is operated as an LC resonant circuit, and the proper communication distance between the transponder and the reader/writer can be set by adjusting resonant frequencies of these coils to a carrier frequency used for communication to allow these coils to be resonant. In the following description, each coil of the transponder and the reader/writer is also referred to as an “antenna”.
The RFID system is classified into three types: a close coupled type (higher than 0 and lower than 2 mm), a proximity type (higher than 0 and lower than 10 cm) and a vicinity type (higher than 0 and lower than 70 cm) according to the transmission distance, which are prescribed by international standards such as ISO/IEC15693, ISO/IEC14443 and ISO/IEC15693, respectively. Among them, as contactless-proximity type IC card standards complying with ISO/IEC14443, Type A, Type B and Felica (trademark of Sony Corporation) can be cited. Type A corresponds to Mifare (trademark) of Royal Phillips Electronics. The card and the reader/writer as SmartCard are standardized as IS07816.
Moreover, NFC (Near Field Communication) developed by Sony Corporation and Royal Phillips Electronics is an RFID standard prescribing specifications of an NFC communication device (reader/writer) which can communicate with respective IC cards of the above Type A, Type B and Felica mainly, which has become the international standard as ISO/IEC IS18092 on December 2003. The NFC communication system takes over “Felica” of Sony Corporation and “Mifare” of Royal Phillips Electronics which have been widely used as a contactless IC card originally, which realizes the proximity-type contactless interactive communication of approximately 10 cm by using 13.56 MHz band in the electromagnetic induction method (NFC prescribes passive communication between the reader/writer and the reader/writer in addition to the communication between the card and the reader/writer).
Presently, NFC is extensively used for individual authentication, electronic money payment and the like. For example, an NFC communication device having an active mode in addition to the passive mode is proposed (for example, refer to JP-A-2005-168069 (Patent Document 1)).
A transfer direction, communication speed, a modulation method and an encoding method according to the communication mode in an NFC IP-1 (interface and Protocol-1) standard are shown in the following chart 1.
TABLE 1A typeB typeFelicaTransferspeed communicationdirection106 kbps106 kbps212 kbps424 kbpsreader/carrier13.5613.5613.56writerfrequencyMHzMHzMHz↓modulation100%10%8-30%cardmethodASKASKASKencoding deformedNRZManchestermethodmirrorcardsubcarrier13.56 MHz/13.56 MHz/—↓frequency−16−16reader/modulationloadload>12% ASKwritermethodmodulationmodulationencoding ManchesterBPSK-ManchestermethodNRZ-L
In the electromagnetic-coupling type contactless communication specifications prescribed by ISO 18092, a Manchester code is used in Felica. In a Felica format, the same packet is used in downlink and uplink. In FIG. 15, a packet structure of the Felica format is shown. The shown packet includes three parts: “preamble”, “sync” and “data”. The preamble has a “0” series of the 6-byte length, the sync has a known series “0xB24D” of 2-byte. The data has LEN of 1-byte indicating the packet length, a data body (payload) of (LEN-1) byte length and a CRC (Cyclic Redundancy Check) code of 2-byte. All these three parts are Manchester encoded.
Here, in the Manchester encoding, when a binary value “0” is sent, the value is changed from the low level to the high level (change the input “0” to “01”) at the center of a bit section, whereas, when a binary value “1” is sent, the value is changed from the high level to the low level (change the input “1” to “10”) at the center of a bit section. In other words, one bit section is divided into a forward cell and a backward cell at the center, and when the forward cell is the low level as well as the backward cell is the high level, a logical value is set to “0” and when the forward cell is the high level and the backward cell is the low level, the logical value is set to “1” in this coding format. In the Manchester encoding, DC components of the transmission signal are cancelled by widening the signal to a double-wide band (converting input 1-bit to 2-bit).
In the preamble part, 6-byte “0” is Manchester encoded. Therefore, the part will be a continuous waveform in which “01” continues 48 times. The sync part has a pattern in which “0xB24D” is Manchester encoded. The data part is Manchester encoded by joining transmission information to Length information (LEN) and CRC.
In the receiving side of the packet, extraction of a clock (sampling timing) is performed based on the preamble part which is the continuous waveform. In the specification, the operation is referred to as “timing synchronization”. Next, the sync part having the pattern in which “0xB24D” is Manchester encoded is detected to estimate a start position of the following data part. In the specification, the operation is referred to as “frame synchronization”. Then, decoding of the data part is performed based on the start position.
In the Felica format, 424 kbps, 848 kbps, 1.7 Mbps, 3.4 Mbps and the like which are multiples of 212 kbps are prescribed as communication rates. As the communication rate is increased, the frequency band of a transmission signal is widened proportionally. As the frequency band of the signal is widened, effects of frequency characteristics in a channel, a transmission RF analog circuit and a reception RF analog circuit are increased. In the frequency characteristics, the attenuation is commonly increased as the frequency becomes higher. The disorder of phase characteristics are also increased as the frequency becomes higher. Accordingly, the higher the communication rate of the signal is, the more marked the disorder of the received waveform becomes.
As a method of compensating the disorder of a received signal in high-speed communication, adaptive equalization processing can be cited. An adaptive equalization circuit includes a FIR (Finite Impulse Response) filter and a learning circuit as an example. A structure of the FIR filter is schematically shown in FIG. 16. The FIR filter includes a delay line in which plural delay elements are connected in series, which weights time-series input data for the numbers of arranged delay elements by tap coefficients in accordance with characteristics of the filter by multipliers respectively, then, accumulates and averages the data to obtain an equivalent signal. After that, the tap coefficients of the filter are determined so that the equivalent signal outputted from the FIR filter comes close to a desired signal by referring to a known training signal (for example, refer to JP-A-2004-64681 (Patent Document 2) and JP-A-2008-22422 (Patent Document 3)).
For example, an RFID system which reduces skew in a radio-wave propagation path by applying an adaptive equalizer as well as reduces communication errors generated by the skew is proposed (refer to JP-A-2008-27270 (Patent Document 4).
In order to perform adaptive equalization, transmission of a random pattern series having the enough length for learning the tap coefficient of the FIR filter will be necessary. On the other hand, in order to decode the data part in the packet from the head, it is necessary to complete learning of the FIR filter in a stage previous to the transmission.
In order to complete the learning of the FIR filter before the data part is arrived, a method of inserting a random pattern which is sufficiently long for learning between the sync part and the data part, a method of transmitting a dedicated packet for learning before a normal packet and the like can be considered. However, in order to realize these methods, a packet format which is different from the Felica format prescribed by the NFC IP-1 standard is used, therefore, a problem concerning compatibility may occur. Since time for transmitting information is reduced for time of transmitting the random pattern for learning as a known signal, the communication rate is likely to be reduced.
In the case of the communication system performing data transmission by load modulation of an antenna such as the NFC communication, transient response characteristics may differ according to a change direction of electrical load (namely, a direction in which load resistance of the antenna is turned on from off and a direction in which it is turned off from on) when performing load modulation of the antenna. FIG. 17 shows amplitude variation of a demodulated signal (preamble part) obtained on the side of the other party of communication when load modulation is performed with respect to a carrier signal generated by the other party of communication. As shown in the drawing, a signal waveform is not symmetric at the rising edge and at the falling edge. Accordingly, it can be considered that it is difficult to obtain sufficient improvement of reception characteristics even when adaptive equalization using the normal FIR filter is performed.
It is desirable to provide an excellent communication device, a communication method, a computer program and an adaptive equalizer suitably capable of solving disorder of a received waveform caused by speeding-up of a communication rate by adaptive equalization.
It is further desirable to provide an excellent communication device, a communication method, a computer program and an adaptive equalizer capable of performing adaptive equalization of a received waveform while keeping compatibility of a packet format based on the NFC standard.
It is further desirable to provide an excellent communication device, a communication method, a computer program and an adaptive equalizer capable of improving reception characteristics by compensating disorder of a received waveform suitably by adaptive equalization in a contactless communication system using load modulation of an antenna even when difference occurs in transient response characteristics of the received-signal waveform according to a change direction of load.